Hiding images in halftone pictures

A method implemented on an encoding device for hiding within a visible representation of a first plea, at least one second plea gray level, the method comprises the following steps: (i) selecting a predetermined key function (ii ) use this key function to create a binary halftone representation of the first plea, in which the first ground is visible and at least one second is hidden; each cell in said binary halftone representation with a distribution of the dot position corresponding to the distribution of gray level of at least one second ground fabric, allowing processing said binary halftone representation by a two-dimensional spatial correlator based on the function extract key information representative of at least a second reason gray level.